A $75,000 salary divided by 26 pay periods yields $2,884.62. The deposit that hits the checking account is usually closer to $1,900. The gap is not friction or fees; it is the result of three different tax bases stacking onto the same gross, in a specific order, where one base — payroll tax — refuses to be reduced no matter what an employee does.
This is the part most paycheck explanations miss. Personal-finance advice frames pre-tax 401(k) contributions as “tax savings,” which implies they reduce all the taxes. They don’t. They reduce two out of three.
The mechanism: stacked bases, not stacked rates
A paycheck is not run through a single tax rate. It is run through three calculations that each pick a slightly different base to operate on.
Base one — gross pay, full amount. The Federal Insurance Contributions Act splits its own tax in two: Social Security at 6.2% and Medicare at 1.45%, totaling 7.65%. Both rates apply to gross pay before any pre-tax deduction. Social Security stops above an annual wage cap; the Social Security Administration projects the 2026 cap at roughly $181,800. Medicare has no cap.
Base two — gross minus pre-tax deductions. Federal income tax withholding does not use gross pay. It uses the gross after subtracting pre-tax 401(k) contributions, traditional HSA contributions, and pre-tax health-insurance premiums. The IRS Publication 15 (Circular E) tables operate on this reduced figure, not on the full salary. Most states follow the same logic for state income tax, though a handful (Pennsylvania, New Jersey for 401(k)) tax some pre-tax contributions anyway.
Base three — the same reduced figure, state-specific. State income tax stacks on the same federal taxable base in most jurisdictions. Some states have no income tax; some are flat; some are progressive. The rate matters less than the base.
The order produces a result that surprises people who increase their 401(k) contribution and watch their take-home drop by less than they expected. A 5% 401(k) contribution on $2,884 reduces federal and state income tax, but the 7.65% FICA hit is calculated on the original $2,884 regardless. That is why pre-tax retirement saving is not a complete tax shield. It is an income-tax shield only.
The math, with a real example
A standard single filer in a 5% state income tax jurisdiction, with a 5% pre-tax 401(k) contribution and an employer health plan, looks like this:
| Line | Base used | Amount |
|---|---|---|
| Gross pay | — | $2,884.62 |
| FICA Social Security (6.2%) | gross | $178.85 |
| FICA Medicare (1.45%) | gross | $41.83 |
| Pre-tax 401(k) (5%) | gross | $144.23 |
| Pre-tax health insurance | gross | $150.00 |
| Federal taxable base | gross − $294.23 pre-tax | $2,590.39 |
| Federal income tax withholding | federal taxable | ~$350.00 |
| State income tax (5%) | federal taxable | ~$129.52 |
| Net deposit | ~$1,890 |
Two things in this table do most of the work. First, the federal taxable base is $2,590.39 — not the $2,884.62 gross. The 401(k) and health-insurance lines reduced it. Second, the FICA lines on rows two and three are still calculated on the full $2,884.62, ignoring the $294.23 in pre-tax deductions sitting right below them.
That $294.23 in pre-tax deductions saves roughly $69 in federal and state income tax in this period (≈22% combined marginal rate × $294.23 of avoided income). It saves exactly $0 in FICA. The 401(k) contribution is not a free 30% tax cut; it is a 22% tax cut.
When the rule of thumb breaks
The 65%-to-75% take-home range is a band, not a constant. Several specific situations shift it.
Earnings above the Social Security cap. Once year-to-date wages cross the 2026 cap (~$181,800), the 6.2% Social Security portion stops mid-year. A high earner’s late-year paychecks are noticeably larger than early-year ones, even though the gross is unchanged. The 1.45% Medicare tax persists, plus an additional 0.9% Medicare surtax above $200,000.
Supplemental income. Bonuses, severance, and most overtime are subject to a flat 22% federal withholding rate under IRS rules, not the employee’s marginal bracket. A $10,000 bonus on top of a $75,000 salary is withheld at 22% — frequently higher than the recipient’s actual marginal rate. The difference comes back at tax-filing time, but the in-period take-home looks punitive.
Roth 401(k) instead of traditional. A Roth contribution does not reduce the federal taxable base. Switching from a 5% traditional contribution to a 5% Roth contribution at the same dollar amount drops the take-home in the current period — the contribution is now taxed before it leaves the paycheck.
No state income tax. Texas, Florida, Tennessee, and six other states eliminate the third base entirely. Removing the $129.52 state line from the table above pushes take-home above 70%.
Decision frame
The right question is not “how much tax am I paying” but “which base is the deduction working on.” A 401(k) contribution and a Roth IRA contribution both reduce taxes over a lifetime, but only one reduces taxes in the current paycheck — and even that one leaves the FICA base untouched. Knowing which dial moves which base is the difference between a budget that fits the paycheck and a budget that assumes the paycheck is bigger than it is.